apt and update-manager disagree

Question

update-manager says there are no updates, but sudo apt --dry-run dist-upgrade says that the sudo package needs to be updated.

$ sudo apt --dry-run dist-upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
  linux-headers-4.8.0-56 linux-headers-4.8.0-56-generic linux-image-4.8.0-56-generic linux-image-extra-4.8.0-56-generic linux-signed-image-4.8.0-56-generic
Use 'sudo apt autoremove' to remove them.
The following packages will be upgraded:
  sudo
1 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Inst sudo [1.8.16-0ubuntu1.4] (1.8.16-0ubuntu1.5 Ubuntu:16.04/xenial-updates [amd64])
Conf sudo (1.8.16-0ubuntu1.5 Ubuntu:16.04/xenial-updates [amd64])

But update-manager says: The software on this computer is up to date.

The system is:

$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.2 LTS"

Is this ever expected behaviour, or is there indeed something weird going on?

I'll hold off for a day or so on doing the update, in case more details are needed.

Thanks.